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Assuming you are referring to MS SQL Server and a typical install, the files are located in program files\Microsoft SQL Server\MSSQL\DATA as MDF files, along with associated LDF log files.

CAUTION!!! Do not simply move them etc., because SQL Server will then not be able to find them. Use the tools within SQL Server if you need to do anything with the files.
You may want to look in the online help to read about detaching/attaching DB files (ie using sp_detach_db).